An MS/MS analysis or MSn analysis, which is a technique of mass spectrometry, is a useful technique for identifying a high-molecular compound and/or analyzing its chemical structure. In recent years, this technique has been widely used in various areas. A commonly known type of mass spectrometer for performing an MS/MS analysis is a triple quadrupole mass spectrometer including two quadrupole mass filters respectively placed before and after a collision cell in which a CID process is performed. A Q-TOF mass spectrometer, which has a time-of-flight mass analyzer in place of the rear quadrupole mass filter in the triple quadrupole mass spectrometer, is more complex in structure and more expensive than the triple quadrupole mass spectrometer yet is capable of acquiring more accurate mass spectra. With these types of mass spectrometers, an MSn analysis with n being two can be performed. Some types of mass spectrometers, such as an ion trap time-of-flight mass spectrometer in which an ion trap capable of holding ions is combined with a time-of-flight mass spectrometer, can perform an MSn analysis with n being equal to or greater than three by dissociating an ion having a specific mass-to-charge ratio in a plurality of stages in the ion trap. In the following description, a mass spectrometer capable of an MSn analysis with n being equal to or greater than two is simply called the mass spectrometer.
In such a mass spectrometer, for example, when an MS2 spectrum for a target component needs to be obtained, it is normally necessary to previously set the mass-to-charge ratio of a precursor ion to be selected as the target in the measurement conditions. However, in the case of analyzing a sample which contains unknown components, it is impossible to previously set the mass-to-charge ratio of the precursor ion. To address this problem, a commonly known mass spectrometer is provided with the function of determining whether or not a peak which meets a precursor-ion selection condition previously set by an analysis operator is present on a mass spectrum obtained by a normal mode of mass spectrometry, and automatically performing an MS2 analysis in real time using the ion corresponding the peak as the precursor ion if a peak which meets the aforementioned condition has been located. Such a function is called the “data dependent acquisition (DDA)” or “auto-MSn analysis” (see Patent Literature 1 or 2). In the present description, “DDA” is used to refer to this function.
As the precursor-ion selection condition in the DDA, the lower-limit threshold of the signal intensity is frequently used. In this case, if a peak whose signal intensity is equal to or higher than a previously set lower-limit threshold is detected on a mass spectrum obtained by a mass spectrometric analysis, an MSn analysis in which the ion corresponding to that peak is selected as the precursor ion will be performed. However, in that case, if there is an ion originating from a foreign substance which is abundantly present, such as the sample solvent, a useless MS2 analysis in which that ion is selected as the precursor ion may be performed. Accordingly, the device described in Patent Literature 2 determines whether or not the signal intensity of a peak which appears on a mass spectrum falls within a range defined by the lower and upper limits specified by an analysis operator, and performs an MS2 analysis using an ion corresponding to a peak whose signal intensity falls within that range. In this case, if the upper-limit value of the signal intensity is appropriately set, it is possible to avoid a component having a high signal intensity for which no MS2 analysis is necessary, and thereby give priority to the MS2 analysis of a component contained at a comparatively low concentration.
However, if a precursor-ion selection as just described is performed in an LC-MS or GC-MS using the previously described mass spectrometer as the detector for a liquid chromatograph (LC) or gas chromatograph (GC), the following problem occurs:
In an LC-MS or GC-MS, the concentration of the target component introduced into the mass spectrometer changes in a bell-like form with the passage of time. Therefore, if the lower limit of the signal intensity in the precursor-ion selection condition is set at an extremely low level, the MS2 analysis will be performed at a point in time where the signal intensity of the target component is not sufficiently high. This may result in an MS2 spectrum showing product ions with low signal intensities and causing a problem in the qualitative determination or structural analysis of the target component. This problem can be avoided by setting the lower limit of the signal intensity at a certain level. However, in that case, the MS2 analysis for the target component may not be performed when the concentration of the target component is lower than expected and does not reach the lower limit of the signal intensity.
Furthermore, in the device described in Patent Literature 2, no MS2 analysis will be performed for an ion if the signal intensity of the corresponding peak observed on the mass spectrum has exceeded the upper limit specified in the precursor-ion selection condition. This is mainly aimed at preventing the MS2 analysis from being performed for a substance which is almost constantly introduced at a high concentration, such as a substance existing in the mobile phase used in an LC or a standard substance used for the mass correction of the mass spectrum. However, it may cause the problem that an MS2 analysis for a substance which the analysis operator wishes to observe will not be performed if the substance is present at such a high concentration that the ions originating from that substance do not meet the precursor-ion selection condition.